Shaped ceramic composites and methods of making the same

ABSTRACT

Self-supporting ceramic composite bodies of desired shape are produced by infiltrating a permeable self-supporting preform with polycrystalline matrix material comprising an oxidation reaction product obtained by oxidation of a parent metal precursor, such as aluminum, and optionally containing therein metallic constituents. The composite body is formed by contacting a zone of a permeable self-supporting preform, having at least one defined surface boundary spaced from said contacting zone, with a body of molten metal which is reacted with a suitable vapor-phase oxidant to form an oxidation reaction product. Within a certain temperature region, and optionally with one or more dopants in or on the parent metal or said permeable preform, molten parent metal migrates through previously formed oxidation reaction product into contact with the oxidant, causing the oxidation reaction product to grow into the preform toward said defined surface boundary so as to infiltrate the preform up to said defined surface boundary with the oxidation reaction product, thus providing the composite structure of desired geometry.

This is a continuation of copending application Ser. No. 07/368,484filed on Jun. 19, 1989, now abandoned, which is a Rule 62 continuationof U.S. patent application Ser. No. 07/109,972, filed on Oct. 19, 1987,now abandoned, which is a division of U.S. patent application Ser. No.06/861,025, filed on May 8, 1986, now abandoned.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates broadly to shaped, self-supporting ceramiccomposites and to methods for producing the same. More particularly,this invention relates to shaped, self-supporting ceramic compositescomprising a shaped preform infiltrated by a ceramic matrix; and tomethods of making novel ceramic composites by infiltrating a shapedpreform with a ceramic matrix by "growing" an oxidation reaction productfrom a parent metal precursor, which product embeds constituents of saidpreform thereby forming a composite having the geometry of said preform.

Description of Commonly Owned Patent Applications and Patents

The subject matter of this application is related to commonly owned U.S.Pat. No. 4,851,375 which issued on Jul. 25, 1989 from U.S. patentapplication Ser. No. 819,397, filed Jan. 17, 1986, which is acontinuation-in-part of U.S. patent application Ser. No. 697,876, filedFeb. 4, 1985, now abandoned, both in the names of Marc S. Newkirk et aland both entitled "Composite Ceramic Articles and Methods of MakingSame." These applications and patents disclose a novel method forproducing a self-supporting ceramic composite by growing an oxidationreaction product from a parent metal into a permeable mass of filler.The resulting composite, however, has no defined or predeterminedgeometry, shape, or configuration.

The method of growing a ceramic oxidation reaction product is disclosedgenerically in commonly owned U.S. Pat. No. 4,713,360 which issued onDec. 15, 1987 and was based on U.S. application Ser. No. 818,943, filedJan. 15, 1986, which was a continuation-in-part of Ser. No. 776,964,filed Sept. 17, 1985, now abandoned which was a continuation-in-part ofSer. No. 705,787, filed Feb. 26, 1985, now abandoned which was acontinuation-in-part of Ser. No. 591,392, filed Mar. 16, 1984, all inthe names of Marc S. Newkirk et al and entitled "Novel Ceramic Materialsand Methods of Making the Same. This method of using an oxidationphenomenon, which may be enhanced by the use of an alloyed dopant,affords self-supporting ceramic bodies grown as the oxidation reactionproduct from a precursor metal. This method was improved upon by the useof dopants applied to the surface of the precursor metal as disclosed incommonly owned U.S. Pat. No. 4,853,352 which issued on Aug. 1, 1989 fromU.S. patent application Ser. No. 822,999, filed Jan. 27, 1986, nowabandoned which is a continuation-in-part of Ser. No. 776,965, filedSep. 17, 1985, now abandoned which is a continuation-in-part of Ser. No.747,788, filed Jun. 25, 1985, now abandoned which is acontinuation-in-part of Ser. No. 632,636, filed Jul. 20, 1984, nowabandoned entitled "Methods of Making Self-Supporting Ceramic Materials," all in the names of Marc S. Newkirk et al. The entire disclosures ofall of the foregoing commonly owned patent applications and patents areexpressly incorporated herein by reference.

Description of the Prior Art

In recent years, there has been an increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,modulus of elasticity, and refractory capabilities when compared withmetals.

Current efforts at producing higher strength, more reliable, and tougherceramic articles are largely focused upon (1) the development ofimproved processing methods for monolithic ceramics and (2) thedevelopment of new material compositions, notably ceramic matrixcomposites. A composite structure is one which comprises a heterogeneousmaterial, body or article made of two or more different materials whichare intimately combined in order to attain desired properties of thecomposite. For example, two different materials may be intimatelycombined by embedding one in a matrix of the other. A ceramic matrixcomposite structure typically comprises a ceramic matrix which enclosesone or more diverse kinds of filler materials such as particulates,fibers, rods or the like.

The traditional methods of preparing ceramic articles involve thefollowing general steps: (1) preparation of ceramic material in powderform; (2) grinding or milling of powders to obtain very fine particles;(3) formation of the powders into a body having the desired geometry(with allowance for shrinkage during subsequent processing), for exampleby uniaxial pressing, isostatic pressing, injection molding, tapecasting, slip casting or any of several other techniques; (4)densification of the body by heating it to an elevated temperature suchthat the individual powder particles merge together to form a coherentstructure, preferably accomplished without the application of pressure(i.e., by pressureless sintering), but in some cases an additionaldriving force is required and can be provided through the application ofexternal pressure either uniaxially (i.e., hot pressing) orisostatically (i.e., hot isostatic pressing); and (5) finishing,frequently by diamond grinding, is required.

When these traditional methods are applied to the preparation of ceramicmatrix composite materials, additional difficulties arise. Perhaps themost serious problems concern the densification step, number (4) above.The normally preferred method, pressureless sintering, can be difficultor impossible in preparing particulate composites if the materials arenot highly compatible. More importantly, conventional sintering isimpossible in most cases involving fiber composites even when thematerials are compatible, because the merging together of the particlesis inhibited by the fibers which tend to prevent the necessarydisplacement of the densifying powder particles. These difficulties havebeen, in some cases, partially overcome by forcing the densificationprocess through the application of external pressure at hightemperature. However, such procedures can generate many problems,including breaking or damaging of the reinforcing fibers by the externalforces applied, limited capability to produce complex shapes (especiallyin the case of uniaxial hot pressing), and generally high costsresulting from low process productivity and the extensive finishingoperations sometimes required.

Additional difficulties also can arise in the blending of powders withwhiskers or fibers, and in the body formation step, number (3) above,where it is important to maintain a uniform distribution of thecomposite second phase within the matrix. For example, in thepreparation of a whisker-reinforced ceramic composite, the powder andwhisker flow processes involved in the mixing procedure, and in theformation of the body, can result in non-uniformities and undesiredorientations of the reinforcing whiskers, with a consequent loss ofperformance characteristics.

A method for producing metal oxide refractories by theoxidation/reduction ("redox") reaction of a metal with silica isdisclosed in U.S. Pat. No. 2,702,750. According to this patent, a silicabody is either submerged into a molten bath of a metal such as aluminum,or a metal powder is dispersed throughout the silica body and thenheated. Where desired, an inert material, such as alumina, may be addedto the body. The refractory product is produced by oxidizing the metalto its oxide while reducing the silica to liberate silicon. In U.S. Pat.No. 3,973,977, there is disclosed a method of making a cermet composedoverwhelmingly of magnesium aluminate spinel by immersing an agglomerateof several oxides into a bath of molten aluminum. Neither of these twopatents discloses the directional growth of an oxidation reactionproduct formed by oxidation of a metal precursor with a vapor-phaseoxidant, nor do they disclose such growth and infiltration into a shapedpreform.

The commonly owned patent applications describe new processes whichresolve some of these problems of traditional ceramic technology asdescribed more fully therein. The present invention combines theseprocesses with additional novel concepts to remove a further limitationof ceramic technology, namely, the formation of complex structures tonet or near net shape. More particularly, the present invention providesfor the formation of composite shapes having a relatively complicatedgeometry or configuration, for example, with contoured planes orsurfaces and with bores or openings. Further, the present inventionprovides for fabrication of ceramic composites of certain predeterminedgeometry by an unusual oxidation phenomenon which overcomes thedifficulties and limitations associated with known processes. Thismethod provides shaped ceramic bodies typically of high strength andfracture toughness by a mechanism which is more direct, more versatileand less expensive than conventional approaches.

The present invention also provides means for reliably producing ceramicarticles as one-piece bodies having a predetermined shape, and of a sizeand thickness which are difficult or impossible to duplicate with thepresently available technology.

SUMMARY OF THE INVENTION

The present invention broadly provides a method for producing aself-supporting ceramic composite body of a predetermined shapecomprising a preform infiltrated by a ceramic matrix. The ceramic matrixis obtained primarily by the oxidation reaction of a parent metalprecursor with a vapor-phase oxidant to form a polycrystalline material,which infiltrates the preform, and consists essentially of the resultingoxidation reaction product and, optionally, one or more metallicconstituents. The vapor-phase oxidant may be used in conjunction witheither a solid oxidant or a liquid oxidant, as explained below ingreater detail, and in such a case the polycrystalline matrix mayinclude the reaction product of the metal precursor with such additionaloxidants and oxidized or reduced constituents of such oxidants. Theresulting self-supporting composite has the configuration or geometrysubstantially that of the preform. The process of the present inventionprovides net or near net shapes which minimizes or eliminates the needfor further shaping or finishing, e.g. by grinding. Also, the productsexhibit such desirable characteristics as straightness, concentricityand general design fidelity.

In accordance with the method of the present invention, the ceramiccomposite is produced by forming at least one permeable preform of adesired, predetermined shape and having at least one defined surfaceboundary. The preform is permeable to the vapor-phase oxidant and toinfiltration by the developing oxidation reaction product. The parentmetal is heated to a temperature above its melting point but below themelting point of the oxidation reaction product to form a body of moltenmetal, but it should be understood that the operable temperature rangeor preferred temperatures may not extend over this entire temperatureinterval. The body of molten metal is contacted with a zone of thepermeable preform, as by positioning the metal adjacent the preform,such that the defined surface boundary of the preform is situatedoutwardly, or away from, or spaced from, the contacting zone, andformation or growth of the oxidation reaction product occurs into thepreform and in a direction towards the defined surface boundary. At thistemperature, or within the temperature range, the molten metal reactswith the oxidant to form a layer of oxidation reaction product. Uponcontinued exposure to the oxidizing environment, and with at least aportion of the oxidation reaction product maintained in contact with andbetween the body of molten metal and the oxidant, molten metal isprogressively drawn through the oxidation reaction product towards theoxidant. In this manner, the oxidation reaction product continues toform at the interface between the oxidant and previously formedoxidation reaction product that has infiltrated the preform. Thereaction is continued until the polycrystalline oxidation reactionproduct has infiltrated the preform to the defined surface boundary, andthus the resulting polycrystalline matrix has embedded the constituentsof the preform in order to produce the ceramic composite having theconfiguration or geometry of the preform.

In another aspect of the invention, there is provided a self-supportingceramic composite body having the configuration or geometrysubstantially that of a shaped preform infiltrated by a ceramic matrixformed upon oxidation of a parent metal precursor, as described below ingreater detail.

The materials of this invention can be grown with substantially uniformproperties throughout their cross-section to a thickness heretoforedifficult to achieve by conventional processes for producing denseceramic structures. The process which yields these materials alsoobviates the high costs associated with conventional ceramic productionmethods, including fine, high purity, uniform powder preparation, greenbody forming, binder burnout, and densification by sintering, hotpressing and/or hot isostatic pressing.

The products of the present invention are adaptable or fabricated foruse as articles of commerce which, as used herein, is intended toinclude, without limitation, industrial, structural and technicalceramic bodies for such applications where electrical, wear, thermal,structural, or other features or properties are important or beneficial;and is not intended to include recycle or waste materials such as mightbe produced as unwanted by-products in the processing of molten metals.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more metallic constituents derived fromthe parent metal or produced from the oxidant or by dopant, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein a metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of the reaction of one or more metals with anoxidant.

"Oxidant" means one or more suitable electron acceptors or electronsharers and may be an element, a combination of elements, a compound, ora combination of compounds, including reducible compounds, and is vapor,solid or liquid at the process conditions.

"Parent metal" refers to that metal, e.g., aluminum, which is theprecursor for the polycrystalline oxidation reaction product, andincludes that metal as a relatively pure metal, a commercially availablemetal with impurities and/or alloying constituents, or an alloy in whichthat metal precursor is the major constituent: and when a specifiedmetal is mentioned as the parent metal, e.g., aluminum, the metalidentified should be read with this definition in mind unless indicatedotherwise by the context.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a parent metal ingot overlaidby an assembly of two preforms forming a rectangular aperture, with thepreform assembly and the metal positioned in an inert bed contained in acrucible, in order to form a complex composite by the method of thisinvention.

FIG. 2(a) is a plan view of a preform shaped as a sprocket for use inproducing the composite according to the present invention.

FIG. 2(b) is a cross-sectional view of the preform of FIG. 2 on line2b--2b of FIG. 2(a).

FIG. 3 is a cross-sectional view showing an assembly of the preform ofFIG. 2(a) overlaying a parent metal.

FIG. 4 is the assembly of FIG. 3 placed in an inert bed contained in acrucible.

FIG. 5 is a photograph of the resulting composite.

FIGS. 6(a) and (b) are photographs of cross-sectional pieces ofcomposites made in accordance with Example 2.

FIGS. 7(a) and (b) are elevational and plan photographs of a preformused in producing the composite of Example 3.

FIG. 7(c) is a photograph of the composite made in accordance withExample 3.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with the present invention, the parent metal, which may bedoped (as explained below in greater detail) and is the precursor to theoxidation reaction product, is formed into an ingot, billet, rod, plate,or the like, and placed in an inert bed, crucible or other refractorycontainer. A permeable, shaped preform (described below in greaterdetail) is manufactured such as to have at least one defined surfaceboundary and to be permeable to the vapor-phase oxidant and to theinfiltrating oxidation reaction product. The preform is placed adjacentto and preferably in contact with one or more surfaces of, or a portionof a surface of, the parent metal such that at least a portion of thedefined surface boundary of the preform is generally positioneddistantly or outwardly or spaced from the metal surface of the parentmetal. The preform preferably is in contact with an areal surface of theparent metal; but when desired, the preform may be partially immersed,but not totally immersed, in the molten metal because complete immersionwould cut off or block access of the vapor-phase oxidant into thepreform for proper development of the polycrystalline matrix. Formationof the oxidation reaction product will occur in a direction towards thedefined surface boundary. This set-up of parent metal and permeablepreform in a suitable container is placed in a furnace, and theenvironment is supplied with a vapor-phase oxidant and heated to processconditions.

The preform useful in the practice of the invention should besufficiently porous or permeable to allow the vapor-phase oxidant topermeate the preform and contact the parent metal. The preform alsoshould be sufficiently permeable to accommodate the development orgrowth of the oxidation reaction product as a matrix within the preformwithout substantially disturbing, upsetting, or otherwise altering theconfiguration or geometry of the preform.

The vapor-phase oxidant is normally gaseous, or vaporized at the processconditions, which provides an oxidizing atmosphere such as atmosphericair. Typical vapor oxidants include, for example, elements or compoundsof the following, or combinations of elements or compounds of thefollowing, including volatile or vaporizable elements, compounds, orconstituents of compounds or mixtures: oxygen, nitrogen, a halogen,sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium,methane, ethane, propane, acetylene, ethylene, propylene (thehydrocarbons as a source of carbon), and mixtures such as air, H₂ /H₂ Oand CO/CO₂, the latter two (i.e., H₂ /H₂ O and CO/CO₂) being useful inreducing the oxygen activity of the environment relative to desirableoxidizable constituents of the preform. Oxygen or gas mixturescontaining oxygen (including air) are suitable vapor-phase oxidants,with air usually being preferred for obvious reasons of economy. When avapor-phase oxidant is identified as containing or comprising aparticular gas or vapor, this means a vapor-phase oxidant in which theidentified gas or vapor is the sole, predominant or at least asignificant oxidizer of the parent metal under the conditions obtainedin the oxidizing environment utilized. For example, although the majorconstituent of air is nitrogen, the oxygen content of air is normallythe sole oxidizer of the parent metal under the conditions obtained inthe oxidizing environment utilized. Air therefore falls within thedefinition of an "oxygen-containing gas" oxidant but not within thedefinition of a "nitrogen-containing gas" oxidant. An example of a"nitrogen-containing gas" oxidant as used herein and in the claims is"forming gas", which typically contains about 96 volume percent nitrogenand about 4 volume percent hydrogen.

An oxidant which is liquid or solid at the process conditions may beemployed in conjunction with the vapor-phase oxidant. Such additionaloxidants may be particularly useful in enhancing oxidation of the parentmetal preferentially within the preform, rather than beyond itssurfaces. That is, the use of such additional oxidants may create anenvironment within the preform more favorable to the oxidation kineticsof the parent metal than the environment outside the preform. Thisenhanced environment is beneficial in promoting matrix developmentwithin the preform to the boundary and minimizing overgrowth. When asolid oxidant is employed, it may be dispersed through the entirepreform or through a portion of the preform adjacent the parent metal,such as in particulate form and admixed with the preform, or it may beutilized as coatings on the preform particles. Any suitable solidoxidant may be employed depending upon its compatibility with thevapor-phase oxidant. Such solid oxidants may include suitable elements,such as boron or carbon, or suitable reducible compounds, such assilicon dioxide (as a source of oxygen) or certain borides of lowerthermodynamic stability than the boride reaction product of the parentmetal.

If a liquid oxidant is employed, the liquid oxidant may be dispersedthroughout the entire preform or a portion thereof adjacent to themolten metal, provided such liquid oxidant does not prevent access ofthe vapor-phase oxidant to the molten parent metal. Reference to aliquid oxidant means one which is a liquid under the oxidation reactionconditions, and so a liquid oxidant may have a solid precursor, such asa salt, which is molten or liquid at the oxidation reaction conditions.Alternatively, the liquid oxidant may be a liquid precursor, e.g. asolution of a material, which is used to coat part or all of the poroussurfaces of the preform and which is melted or decomposed at the processconditions to provide a suitable oxidant moiety. Examples of liquidoxidants as herein defined include low melting glasses.

The lay-up, comprising the parent metal and permeable preform, is placedin a furnace that is supplied with a vapor-phase oxidant, and the lay-upis heated to or within a temperature region above the melting point ofthe parent metal but below the melting point of the oxidation reactionproduct. For example, in the case of an aluminum parent metal using airas the vapor-phase oxidant, the temperature is generally between about850°-1450° C. and more preferably between about 900°-1350° C. Withinthis operable temperature interval or preferred temperature range, abody or pool of molten metal forms, and on contact with the oxidant(s),the molten metal will react to form a layer of oxidation reactionproduct. Upon continued exposure to the oxidizing environment, within anappropriate temperature region, the remaining molten metal isprogressively drawn into and through the oxidation reaction product inthe direction of the oxidant. On contact with the oxidant, the moltenmetal will react to form additional oxidation product. At least aportion of the oxidation reaction product is maintained in contact withand between the molten parent metal and the vapor-phase oxidant tosustain the continued growth of the polycrystalline oxidation reactionproduct in the preform. The polycrystalline reaction product willcontinue to grow and develop within the preform, embedding itsconstituents, generally if there is provided an interchange ofvapor-phase oxidant, and additional parent metal is present. When thevapor-phase oxidant is air, this interchange is effected mostconveniently by providing venting means within the furnace.

The process is continued until the oxidation reaction product hasembedded the constituents of the preform to the defined surfaceboundary, and desirably not beyond, which would be an "over-grow" by thepolycrystalline matrix material. The resulting ceramic composite productincludes a preform infiltrated to its boundaries by a ceramic matrixcomprising a polycrystalline material consisting essentially of theoxidation reaction product of the parent metal with the vapor-phaseoxidant and, optionally, one or more metallic constituents such asnon-oxidized constituents of the parent metal or metallic constituentsof a reducible oxidant. Most typically, the boundaries of the preformand of the polycrystalline matrix substantially coincide; but individualconstituents at the surfaces of the preform may be exposed or mayprotrude from the matrix, and therefore infiltration and embedment maynot completely surround or encapsulate the preform by the matrix. Itfurther should be understood that the resulting polycrystalline matrixmay exhibit porosity which may be a partial or nearly completereplacement of the metal phase, but the volume percent of voids willdepend largely on such conditions as temperature, time, type of parentmetal, and dopant concentrations. Typically in these polycrystallineceramic structures, the oxidation reaction product crystallites areinterconnected in more than one dimension, preferably in threedimensions, and the metal phase or pore phase may be at least partiallyinterconnected. The ceramic composite product of this invention hasgenerally well-defined boundaries and possesses the dimensions andgeometric configuration of the original preform.

Although the present invention is hereinafter described with particularemphasis on aluminum and specific embodiments of aluminum as the parentmetal, this reference is for exemplary purposes only, and it is to beunderstood that other metals such as silicon, titanium, tin, zirconium,etc., also can be employed which satisfy, or can be doped to satisfy,the criteria of the invention. Examples of materials useful infabricating a preform in practicing the present invention, dependingupon the parent metal and oxidation system chosen, may include one ormore of aluminum oxide, silicon carbide, silicon aluminum oxynitride,zirconium oxide, zirconium boride, titanium nitride, barium titanate,boron nitride, silicon nitride, various ferrous alloys, e.g., aniron-chromium-aluminum alloy, carbon, aluminum, various clays, andmixtures thereof. However, any suitable material may be employed in thepreform. For example, if aluminum is employed as the parent metal, andaluminum nitride is the intended oxidation reaction product, aluminumnitride and/or aluminum oxide particles would be suitable materials forthe preform; if zirconium is employed as a parent metal, and zirconiumnitride is the intended oxidation reaction product, zirconium diborideparticles would comprise a suitable composition for a preform; iftitanium is employed as a parent metal and titanium nitride is theintended oxidation reaction product, a preform comprised of aluminaand/or titanium diboride particles would be suitable; if tin is employedas a parent metal, and tin oxide is the intended oxidation reactionproduct, a preform comprised of alumina particles would be suitable; orif silicon is employed as the parent metal and silicon nitride is theintended oxidation reaction product, a preform comprised of titaniumnitride particles would be suitable.

The permeable preform of this invention may be created or formed intoany predetermined or desired size and shape by any conventional method,such as slipcasting, injection molding, transfer molding, vacuumforming, or otherwise, by processing any suitable material(s), whichwill be more specifically identified and described hereafter. Thepermeable preform, as was previously mentioned, may include or haveincorporated therein a solid oxidant and/or a liquid oxidant which maybe used in conjunction with the vapor-phase oxidant. The preform shouldbe manufactured with at least one surface boundary, and should retainsufficient shape integrity and green strength to provide dimensionalfidelity prior to being infiltrated by the ceramic matrix. The permeablepreform, however, should be permeable enough to accommodate the growingpolycrystalline matrix. Preferably, the preforms of this invention havea porosity of between about 5 and 90% by volume, and more preferablybetween about 25 and 50%. The porous preform preferably should becapable of being wetted by the molten parent metal under processtemperature conditions in order to encourage development of thepolycrystalline matrix within the preform to produce a ceramic compositeproduct of high integrity and well-defined borders.

The preform, being of any size or shape, has at least one surfaceboundary which essentially defines the destination or boundary for thegrowing polycrystalline matrix. By way of example only, the preform maybe hemispherical in shape with the flat surface boundary in contact withthe parent metal surface and the dome-shaped surface boundaryrepresenting the defined surface boundary to which the polycrystallinematerial is to grow; or the preform may be cubical in shape with onesquare surface boundary contacting the metal surface of the parent metaland the remaining five square surface boundaries being the objectivepoints for the growing polycrystalline material. A matrix of thepolycrystalline material resulting from the oxidation reaction is growninto the permeable preform so as to infiltrate and embed theconstituents of the latter to its defined surface boundary withoutsubstantially disturbing or displacing it. Thus, no external forces areinvolved which might damage the preform, little or no shrinkage isinvolved which might crack the preform and cause it to lose fidelitywith respect to its original shape and tolerance, and no awkward andcostly high temperature, high pressure processes and facilities arerequired to achieve a composite ceramic product. In addition, thespecial requirements of chemical and physical compatibility necessaryfor pressureless sintering of particulate composites are avoided by thepresent invention.

The permeable preform of this invention may be composed of any suitablematerial, such as ceramic and/or metal particulates, powders, fibers,whiskers, wires, particles, hollow bodies or spheres, wire or refractorycloth, solid spheres, etc., and combinations thereof. The preformmaterials typically comprise a bonded array or arrangement, which arrayhas interstices, openings, intervening spaces, or the like to render thepreform permeable to the oxidant and the infiltration of the oxidationreaction product growth without altering the configuration of thepreform. The preform may include a lattice of reinforcing rods, bars,tubes, tubules, plates, wires, spheres or other particulates, platelets,wire cloth, ceramic refractory cloth or the like, or a combination ofany of the foregoing, prearranged in a desired shape. Further, thematerial(s) of the preform may be homogeneous or heterogeneous. Thesuitable materials of the preform, such as ceramic powders orparticulate, may be bonded together with any suitable binding agent,e.g. polyvinyl alcohol or the like, which does not interfere with thereactions of this invention, or leave undesirable residual by-productswithin the ceramic composite product. Suitable particulates, such assilicon carbide or alumina, having a grit or mesh size of from about 10to 1000 or finer, or an admixture of mesh sizes and types, for example,may be used. The particulate may be molded by known or conventionaltechniques as by forming a slurry of the particulate in an organicbinder, pouring the slurry into a mold, and then letting the part set asby drying at an elevated temperature.

More specifically, with respect to suitable materials that may beemployed in the formation and manufacture of the permeable preform,three classes of materials may be identified as suitable materials forthe permeable preform.

The first class of preform materials includes those chemical specieswhich, under the temperature and oxidizing conditions of the process,are not volatile, are thermodynamically stable and do not react with oro dissolve excessively in the molten parent metal. Numerous materialsare known to those skilled to the art as meeting such criteria in thecase where aluminum as the metal and air or oxygen as the oxidant, forexample, are employed. Such materials include the single-metal oxidesof: aluminum, Al₂ O₃ ; cerium, CeO₂ ; hafnium, HfO₂ ; lanthanum, La₂ O₃; neodymium, Nd₂ O₃ ; praseodymium, various oxides; samarium, Sm₂ O₃ ;scandium, Sc₂ O₃ ; thorium, ThO₂ ; uranium, UO₂ ; yttrium, Y₂ O₃ ; andzirconium, ZrO₂. In addition, a large number of binary, ternary, andhigher order metallic compounds such as magnesium aluminate spinel,MgOAl₂ O₃, are contained in this class of stable refractory compounds.

A second class of suitable materials for the preform are those which arenot intrinsically stable in the oxidizing and high temperatureenvironment, but which, due to the relatively slow kinetics of thedegradation reactions, can act and/or perform as the preform phase wheninfiltrated by the growing polycrystalline ceramic matrix. Aparticularly useful material for this invention is silicon carbide. Thismaterial would oxidize completely under the conditions necessary tooxidize aluminum with oxygen or air in accordance with the inventionwere it not for a protective layer of silicon oxide forming and coveringthe silicon carbide particles to limit further oxidation of the siliconcarbide.

A third class of suitable materials for the preform of this inventionare those which are not, on thermodynamic or on kinetic grounds,expected to survive the oxidizing environment or the exposure to moltenmetal necessary for the practice of the invention. Such a preform can bemade compatible with the process of the present invention if (1) theenvironment is made less active, for example, through the use of H₂ /H₂O or CO/CO₂ mixtures as the oxidizing gas, or (2) through theapplication of a coating thereto, such as aluminum oxide, which makesthe species kinetically non-reactive in the process environment. Anexample of such a class of preform materials would be carbon fiberemployed in conjunction with a molten aluminum parent metal. If thealuminum is to be oxidized with air or oxygen at, for example, 1250° C.,to generate a matrix incorporating a preform containing said fibers, thecarbon fiber will tend to react with both the aluminum (to form aluminumcarbide) and the oxidizing environment (to form CO or CO₂). Theseunwanted reactions may be avoided by coating the carbon fiber (forexample, with alumina) to prevent reaction with the parent metal and/oroxidant and optionally employing a CO/CO₂ atmosphere as oxidant whichtends to be oxidizing to the aluminum but not the contained carbonfiber.

The preform of this invention may be employed as a single preform or asan assemblage of preforms to form more complex shapes. It has beendiscovered that the polycrystalline matrix can grow through adjacent,contacting portions of a preform assemblage, and bond contiguouspreforms at their contact surfaces into a unified or integral ceramiccomposite. The assembly of preforms is arranged so that the direction ofgrowth of the oxidation reaction product will be towards and into theassembly of preforms to infiltrate and embed the assembly to theboundaries defined by the assembled preforms. Thus, complex ceramiccomposites can be formed as an integral body which cannot otherwise beproduced by conventional manufacturing techniques. It should beunderstood that whenever the term "preform" is used herein and in theclaims, it means a single preform or an assemblage of preforms, unlessotherwise stated.

By way of example only of such an assemblage of preforms, FIG. 1 is avertical cross-sectional view of a crucible 10 containing an inert bed12 which includes a parent metal 14 overlaid by an assemblage ofpreforms comprising preform 16 with recess 18, and preform 20 having atop surface boundary 21 and a recess 22. Preform 20 is superimposed ontopreform 16 such that the borders or marginal edges of the recesses 18and 22 register, and the recesses 18 and 22 complement each other anddefine a rectangular aperture 24. The surfaces of recesses 18 and 22 maybe provided with a barrier means, e.g. Plaster of Paris, as describedand claimed in commonly owned U.S. Pat. No. 4,923,832, which issued onMay 8, 1990, from U.S. application Ser. No. 861,024, filed on May, 8,1986, entitled "Method of Making Shaped Ceramic Composites with the Useof a Barrier," in the names of Newkirk et al, and assigned to the sameowner, for inhibiting a growth of the oxidation reaction product beyondthe surfaces of the preform and within aperture 24. As explained above,the polycrystalline matrix is grown to infiltrate the assemblage ofpreforms 16 and 20 to the top surface boundary 21 of preform 20 such asto bond or unify preforms 16 and 20 and produce a ceramic compositehaving the rectangular aperture 24.

In producing a net or near net shape ceramic composite body whichretains essentially the original shape and dimensions of the preform,growth of the ceramic matrix should occur to the at least one definedsurface boundary of the preform. Growth beyond the surface boundariescan be prevented, inhibited or controlled by any one or combination ofthe following steps: (1) creating conditions within the preform, forexample, by incorporating solid or liquid oxidants in the preform suchthat internal growth is highly preferred to growth beyond the preformsurfaces; (2) using a substantially exact, predetermined quantity ofparent metal such that when it is entirely consumed or converted intothe polycrystalline structure, the oxidation reaction product is at theboundary of the permeable preform; (3) controlling or limiting theamount of oxidants available to the process initially; (4) providing abarrier means on the preform surface(s) as described in commonly ownedU.S. Pat. No. 4,923,832 mentioned above; or (5) at the appropriate time,stopping the process by evacuating, or eliminating, the oxidizingatmosphere or by altering the reaction temperature to be outside theprocess temperature envelope, e.g., lowered below the melting point ofthe parent metal. Usually, the temperature is reduced by lowering thefurnace temperature, and then the material is removed from the furnace.Step (5) may require vigilance to avoid matrix overgrowth of any definedsurface boundary.

The ceramic composite product obtained by the practice of the presentinvention will usually be a dense coherent product wherein between about5% and about 98% by volume of the total volume of the ceramic compositeproduct is comprised of one or more of the preform materials embeddedwithin a polycrystalline ceramic matrix. The polycrystalline ceramicmatrix is usually comprised of, when the parent metal is aluminum andair or oxygen is the oxidant, about 60% to about 99% by weight (of theweight of polycrystalline matrix) of interconnected α-alumina oxide andabout 1% to 40% by weight (same basis) of nonoxidized metallicconstituents.

As a further embodiment of the invention and as explained in thecommonly owned patent applications, the addition of dopant materials inconjunction with the metal can favorably influence the oxidationreaction process. The function or functions of the dopant material candepend upon a number of factors other than the dopant material itself.These factors include, for example, the particular parent metal, the endproduct desired, the particular combination of dopants when two or moredopants are used, the use of an externally applied dopant in combinationwith an alloyed dopant, the concentration of the dopant, the oxidizingenvironment, and the process conditions.

The dopant or dopants used in conjunction with the parent metal (1) maybe provided as alloying constituents of the parent metal, (2) may beapplied to at least a portion of the surface of the parent metal, or (3)may be applied to or incorporated into the preform or a part of thepreform, or any combination of two or more of techniques (1), (2) and(3) may be employed. For example, an alloyed dopant may be used incombination with an externally applied dopant. In the case of technique(3), where a dopant or dopants are applied to the preform, theapplication may be accomplished in any suitable manner, such as bydispersing the dopants throughout part or the entire mass of the preformas coatings or in particulate form, preferably including at least aportion of the preform adjacent the parent metal. Application of any ofthe dopants to the preform may also be accomplished by applying a layerof one or more dopant materials to and within the preform, including anyof its internal openings, interstices, passageways, intervening spaces,or the like, that render it permeable. A convenient manner of applyingany of the dopant material is to merely soak the entire bed in a liquid(e.g., a solution) of dopant material or its precursor. A source of thedopant may also be provided by placing a rigid body of dopant in contactwith and between at least a portion of the parent metal surface and thepreform. For example, a thin sheet of silicon-containing glass (usefulas a dopant for the oxidation of an aluminum parent metal) can be placedupon a surface of the parent metal. When the aluminum parent metal(which may be internally doped with Mg), overlaid with thesilicon-containing material, is melted in an oxidizing environment(e.g., in the case of aluminum in air, between about 850° C. to about1450° C., preferably about 900° C. to about 1350° C.), growth of thepolycrystalline ceramic matrix into the permeable preform occurs. In thecase where the dopant is externally applied to at least a portion of thesurface of the parent metal, the polycrystalline oxide structuregenerally grows within the permeable preform substantially beyond thedopant layer (i.e., to beyond the depth of the applied dopant layer). Inany case, one or more of the dopants may be externally applied to theparent metal surface and/or to the permeable preform. Additionally,dopants alloyed within the parent metal and/or externally applied to theparent metal may be augmented by dopant(s) applied to the preform. Thus,any concentration deficiencies of the dopants alloyed within the parentmetal and/or externally applied to the parent metal may be augmented byadditional concentration of the respective dopant(s) applied to thepreform and vice versa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal, incombination with each other or in combination with other dopants asdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. Concentrations within this range appear toinitiate the ceramic growth, enhance metal transport and favorablyinfluence the growth morphology of the resulting oxidation reactionproduct.

Other dopants which are effective in promoting polycrystalline oxidationreaction growth for aluminum-based parent metal systems are, forexample, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of them, is alloyed into the aluminum parent metalsystem at concentrations for each of from about 0.5 to about 15% byweight of the total alloy; however, more desirable growth kinetics andgrowth morphology are obtained with dopant concentrations in the rangeof from about 1-10% by weight of the total parent metal alloy. Lead as adopant is generally alloyed into the aluminum-based parent metal at atemperature of at least 1000° C. so as to make allowances for its lowsolubility in aluminum; however, the addition of other alloyingcomponents, such as tin, will generally increase the solubility of leadand allow the alloying material to be added at a lower temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsinclude (a) magnesium and silicon or (b) zinc and silicon. In suchexamples, a preferred magnesium concentration falls within the range offrom about 0.1 to about 3% by weight, for zinc in the range of fromabout 1 to about 6% by weight, and for silicon in the range of fromabout 1 to about 10% by weight.

Additional examples of dopant materials useful with an aluminum parentmetal when air is employed as an oxidant, include sodium, lithium,calcium, boron, phosphorus and yttrium, which dopants may be usedindividually or in combination with one or more other dopants dependingon the oxidant and process conditions. Sodium and lithium may be used invery small amounts in the parts per million range, typically about100-200 parts per million, and each may be used alone or together, or incombination with other dopant(s). Rare earth elements such as cerium,lanthanum, praseodymium, neodymium and samarium are also useful dopants,and herein again especially when used in combination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe parent metal enables or improves local ceramic growth from theparent metal surface or portions thereof and lends itself to desiredgrowth of the polycrystalline ceramic matrix into the permeable preform.Thus, growth of the polycrystalline ceramic matrix into the permeablepreform can be favorably influenced by the localized placement of thedopant material upon the parent metal surface. The applied coating orlayer of dopant is thin relative to the thickness of the parent metalbody, and growth or formation of the oxidation reaction product into thepermeable preform extends to substantially beyond the dopant layer,i.e., to beyond the depth of the applied dopant layer. Such layer ofdopant material may be applied by painting, dipping, silk screening,evaporating, or otherwise applying the dopant material in liquid orpaste form, or by sputtering, or by simply depositing a layer of a solidparticulate dopant or a solid thin sheet or film of dopant onto thesurface of the parent metal. The dopant material may, but need not,include either organic or inorganic binders, vehicles, solvents, and/orthickeners. More preferably, the dopant materials are applied as powdersto the surface of the parent metal or dispersed through at least aportion of the filler. One particularly preferred method of applying thedopants to the parent metal surface is to utilize a liquid suspension ofthe dopants in a water/organic binder mixture sprayed onto a parentmetal surface in order to obtain an adherent coating which facilitateshandling of the doped parent metal prior to processing.

The dopant materials, when used externally, are usually applied to aportion of a surface of the parent metal as a uniform coating thereon.The quantity of dopant is effective over a wide range relative to theamount of parent metal to which it is applied and, in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as the dopant for an aluminum-basedparent metal using air or oxygen as the oxidant, quantities as low as0.00003 gram of silicon per gram of parent metal, or about 0.0001 gramof silicon per square centimeter of exposed parent metal surface,together with a second dopant providing a source of magnesium and/orzinc produce the polycrystalline ceramic growth phenomenon. It also hasbeen found that a ceramic structure is achievable from an aluminum-basedparent metal using air or oxygen as the oxidant by using MgO as thedopant in an amount greater than 0.0008 gram of dopant per gram ofparent metal to be oxidized and greater than 0.003 gram of dopant persquare centimeter of parent metal surface upon which the MgO is applied.It appears that to some degree an increase in the quantity of dopantmaterials will decrease the reaction time necessary to produce theceramic composite, but this will depend upon such factors as type ofdopant, the parent metal and the reaction conditions.

Where the parent metal is aluminum internally doped with magnesium andthe oxidizing medium is air or oxygen, it has been observed thatmagnesium is at least partially oxidized out of the alloy attemperatures of from about 820° to 950° C. In such instances ofmagnesium-doped systems, the magnesium forms a magnesium oxide and/ormagnesium aluminate spinel phase at the surface of the molten aluminumalloy, and during the growth process such magnesium compounds remainprimarily at the initial oxide surface of the parent metal alloy (i.e.,the "initiation surface") in the growing ceramic structure. Thus, insuch magnesium-doped systems, an aluminum oxide-based structure isproduced apart from the relatively thin layer of magnesium aluminatespinel at the initiation surface. Where desired, this initiation surfacecan be readily removed as by grinding, machining, polishing orgritblasting.

The invention is further illustrated by the following examples which aregiven by way of illustration only and are not intended to be limiting.

EXAMPLE 1

Referring in detail to FIGS. 2-5, wherein the same reference numeralsdesignate similar parts throughout, a ceramic sprocket 38 was fabricatedfrom a preform 30 having the shape shown in FIGS. 2a and 2b. The preformmeasured 3 inches in outer diameter and 3/16 inch thick, and had acenter key hole 31. The preform was prepared by a conventional methodemploying silicon carbide particles. A uniform mixture comprising 80weight percent silicon carbide particles (an admixture of 70 weight %500 grit and 30 weight % 220 grit) and 20 weight percent of an organicbinder solution (in a 4 to 1 ratio of Elmer's wood glue to water) waspoured into a silicone rubber mold, and then dried to set. The rigidsprocket shape 30 was then removed from the rubber mold.

A three-inch diameter cylindrical plate 32 of an aluminum alloydesignated 380.1 (from Belmont Metals Inc., having a nominallyidentified composition by weight of 8-8.5% Si, 2-3% Zn and 0.1% Mg asactive dopants, and 3.5% Cu as well as Fe, Mn, and Ni, but the Mgcontent was sometimes higher as in the range of 0.17-0.18%), alloyedwith an additional 6% lead, was placed in contact with preform surface33. Ingot 34, of the same alloy 380.1 to provide sufficient quantity ofalloy to enable complete infiltration of the preform, was placed incontact with surface 28 of plate 32. The combination of the cylindricalplate and ingot weighed 100 g. The system (preform 30 and alloys 32 and34), set up as shown in FIG. 3, was coated on all exposed surfaces by anaqueous slurry of Plaster of Paris (Bondex containing about 35 weightpercent calcium carbonate, from Bondex International, St. Louis, Mo.) toprevent overgrowth of the preform geometry by the ceramic matrix asdescribed in commonly owned U.S. Pat. No. 4,923,832 (identified above).The Plaster of Paris coating 35 was allowed to set and the coated unitwas completely submerged into a bed 36 of alumina particles (E1 Alundumfrom Norton Company, 90 grit) contained in a refractory crucible 37, asshown in FIG. 4.

The system as shown in FIG. 4 was heated in air from an initialtemperature of 200° C. at a rate of 250° C./hour to a final temperatureof 1000° C. where it was held for 66 hours in air. The furnace was thencooled at the same rate and the sample was removed at approximately 600°C. The procedure resulted in a ceramic composite comprising an α-aluminamatrix (as evinced by X-ray powder diffraction analysis of the material)completely embedding the silicon carbide particles of the sprocket up tothe plaster covered boundaries of the preform. The excess aluminumadhering to the face 33 of the sprocket, and the dehydrated plasterlayer, were mechanically removed from the formed composite. Theresulting ceramic sprocket 38 exhibited a high fidelity duplication ofthe preform, as shown in FIG. 5, and had an average Rockwell A hardnessof 79.8.

EXAMPLE 2

Two preforms measuring 21/4 inches square and 1/4-3/8 inch thick wereprepared comprising 95% by weight alumina particles (E38 Alundum fromNorton Co., 90 mesh size) and 5% by weight silicon dioxide. The preformswere shaped by, first admixing the alumina with an organic binder(Avecil PH-105 from FMC Co.), then pressing the composition into thespecified geometry at 7900 psi, and finally prefiring said preforms at1375° C. for 24 hours. Each of the two preforms was placed on top of abed of alumina particles (E38 Alundum, from Norton, 24 mesh size)contained by a refractory vessel. Two, 2 inch square by 1/2 inch thick,blocks of aluminum having different alloy composition were used as theparent metal, one of each being placed on top of each preform. The twoalloys employed were 99% pure aluminum and 380.1 alloy (having thenominal composition described in Example 1 without the additional 6%lead).

The above two systems were heated to a setpoint temperature of 900° C.in air for 36 hours, a time sufficient for the α-alumina ceramic matrixto completely infiltrate the preform to the opposite, defined boundary.Formation of an α-alumina ceramic matrix was confirmed by X-ray powderdiffraction analysis. FIGS. 6a and 6b show an elevational cross sectionof the ceramic products of the present Example. Upon examination of thebody 45 produced from the 99% pure aluminum, and the body 47 producedfrom the 380.1 alloy, the α-alumina ceramic matrix in each case wasobserved to have penetrated completely into the preform. The overgrowthof the preform boundaries by the ceramic matrix was limited to the faceof the preform exposed to the alumina particle bedding, and varied indegree between the two systems. The sample which employed the 99% purealuminum precursor showed negligible overgrowth of the preform boundaryby the ceramic matrix into the filler bed, which could be easily removedby light machining or grinding. FIG. 6a is illustrative of the verylimited overgrowth 46 of this ceramic composite 45. Since the ceramicmatrix resulting from the oxidation of the 380.1 alloy apparentlyrequired less time to penetrate the preform, for the same reaction time,the ceramic composite 47 had substantial overgrowth 48. Hence, fidelitycan be achieved by controlling the reaction so as not to allow growth ofthe ceramic matrix beyond the defined preform boundary.

EXAMPLE 3

Referring to FIGS. 7a, b, and c, a preform 50 having a trapazoidal shapein elevation (13/4 inches thick and having a rectangular face 51measuring 8 7/16 inches ×21/2 inches and rectangular face 52 measuring85/8 inches by 23/4 inches) was cast, by a conventional method, from amixture comprising 32 weight percent alumina particles (E67 Alundum,from Norton Co., 1000 mesh size), 35 weight percent silicon dioxide (500mesh size), 0.5 weight percent silicon, 0.5 weight percent sodiumsilicate (introduced as a predissolved solute in the water used toslurry the preform mixture as described below) and 32 weight percentGreencast 94 binder (from A. P. Green Refractories Co., Mexico Mo., 100mesh size and finer). The above mixture was slurried in water(containing the above specified amount of dissolved sodium silicate) andpoured into a mold having the described geometry. The mixture wasallowed to air dry and was removed from the mold as a rigid trapazoidalbody 50. The word "Lanxide" was inscribed on face 52 of the preform (seeFIG. 7b), and the preform was fired in air at 1000° C. for 1 hour.

Two bars of commercially available 5052 alloy (having a nominalcomposition by weight of 2.5% Mg and approximately 1% combined total ofother species such as Fe, Cr, Si, and Cu), and one bar of 99% purealuminum, each measuring 81/2 inches long by 21/2 inches wide by 1/2inch thick, were stacked such that the pure aluminum bar was between thetwo 5052 bars; and the stack was placed on top of a thin layer ofsilicon carbide particles (24 mesh size) contained in a refractoryvessel. The trapazoidal preform was placed on top of the stack ofaluminum bars such that face 51 of the preform was entirely in contactwith the top 81/2 inch by 21/2 inch rectangular face of the stack ofaluminum alloy bars, and thus the entire weight of the preform wassupported by the metal stack. The crucible was then filled with siliconcarbide particles (14 mesh size) as to completely cover the aluminummetal but allowing the five surfaces of the preform, not in contact withthe aluminum metal, to remain substantially exposed to the atmosphere.

The above system was placed in a furnace (which was vented to the flowof air) and heated up over a 5 hour period to a reaction temperature of1000° C. The furnace was maintained at that reaction temperature for 144hours. The furnace was cooled to ambient and reheated to 1000° C. for 6additional hours to completely infiltrate the preform.

The molten aluminum metal reacted with the oxidants (vapor phase oxidantand solid oxidants such as silica) forming an α-alumina ceramic matrixwhich infiltrated the preform thus embedding the particles of thepreform composition. Formation of the ceramic matrix continuedcompletely to the surface boundaries of the preform and wassubstantially contained within those defined boundaries. Examination ofthe composite product 53 showed high fidelity compared to the preform asevidenced by the clear imprint (see FIG. 7c), with only negligibleovergrowth by the ceramic matrix.

The foregoing is illustrative of the embodiment of the present inventionwherein the composition of the preform enhances the oxidation of themolten parent metal preferentially within the preform boundaries. Suchpreferential oxidation helps to control overgrowth of the preformboundaries by the ceramic matrix. The body 53 thus obtained is a shapedceramic article maintaining the geometry of the trapazoidal preform 50.

What is claimed is:
 1. A shaped self-supporting ceramic matrix compositebody comprising a ceramic matrix incorporating at least one preform,said preform comprising a shaped body of filler material which iscapable of supporting its own weight and maintaining dimensionalfidelity and interconnected porosity without any means of supportlocated external to any surface of said preform, said ceramic matrixbeing disposed within at least a portion of said interconnected porosityso as to embed the filler material and said ceramic matrix consistingessentially of about 60-99 percent by weight of an essentially singlephase polycrystalline oxidation reaction product consisting essentiallyof a material selected from the group consisting of alumina, aluminumnitride and silicon nitride, and the remainder of said ceramic matrixconsisting essentially of at least one metallic constituent and voids,said self-supporting ceramic matrix composite body correspondingsubstantially in shape to the configuration of said at least onepreform.
 2. The ceramic matrix composite body of claim 1, wherein saidoxidation reaction product consists essentially of alumina or aluminumnitride, and the metallic constituent comprises aluminum.
 3. The ceramicmatrix composite body of claim 1, wherein said metallic constituentcomprises at least one metal dispersed throughout said ceramic matrix,wherein at least a portion of said metallic constituent isinterconnected.
 4. The ceramic matrix composite body of claim 1, whereinsaid metallic constituent comprises at least one metal dispersedthroughout said ceramic matrix, wherein at least a portion of saidmetallic constituent consists of non-interconnected inclusions.
 5. Theceramic matrix composite body of claim 1, wherein said remainder of saidceramic matrix consists essentially of at least one metallic constituentand voids, said voids comprising at least 1% by volume of said ceramicmatrix and said voids being dispersed throughout said ceramic matrix,wherein at least a portion of said voids are interconnected.
 6. Theceramic matrix composite body of claim 1, wherein said oxidationreaction product consists essentially of alpha-alumina.
 7. The ceramicmatrix composite body of claim 1, wherein said oxidation reactionproduct consists essentially of aluminum nitride.
 8. The ceramic matrixcomposite body of claim 1, wherein said oxidation reaction productconsists essentially of alumina and said metallic constituent comprisesaluminum, wherein at least a portion of said metallic constituent isinterconnected.
 9. The ceramic matrix composite body of claim 1, whereinsaid oxidation reaction product consists essentially of aluminum nitrideand said metallic constituent comprises aluminum, wherein at least aportion of said metallic constituent is interconnected.
 10. The ceramicmatrix composite body of claim 1, wherein said oxidation reactionproduct consists essentially of silicon nitride and said metallicconstituent comprises silicon.
 11. The ceramic matrix composite body ofclaim 1, wherein said filler material comprises at least one materialselected from the group consisting of aluminum oxide, silicon carbide,silicon aluminum oxynitride, zirconium oxide, zirconium boride, titaniumnitride, barium titanate, boron nitride, silicon nitride, zirconiumnitride, titanium diboride, and aluminum nitride.
 12. The ceramic matrixcomposite body of claim 1, wherein said oxidation reaction productconsists essentially of alumina, said at least one preform consistsessentially of silicon carbide and said metallic constituent comprisesaluminum.
 13. The ceramic matrix composite body of claim 1, wherein saidoxidation reaction product consists essentially of aluminum nitride,said at least one preform consists essentially of aluminium nitride andsaid remainder of said ceramic matrix consists essentially of a metallicconstituent comprising aluminum and voids.
 14. The ceramic matrixcomposite body of claim 1, wherein said oxidation reaction productconsists essentially of silicon nitride, said at least one preformconsists essentially of silicon nitride and said remainder of saidceramic matrix consists essentially of silicon and voids.
 15. A dense,coherent and shaped self-supporting ceramic matrix composite bodycomprised of from about 5% to about 98% by volume, based on the totalvolume of the ceramic matrix composite body, of at least one preformcontained within a ceramic matrix, said preform being a shaped body offiller material having a configuration which is capable of supportingits own weight and maintaining dimensional fidelity and interconnectedporosity without any means of support located external to any surface ofsaid preform, said ceramic matrix being disposed within at least aportion of said interconnected porosity so as to embed the fillermaterial and said ceramic matrix consisting essentially of, based on thetotal weight of said matrix, about 60% to about 99% by weight of aninterconnected oxidation reaction product consisting essentially ofalumina and the remainder of said ceramic matrix consisting essentiallyof a metallic constituent comprising aluminum and voids, saidself-supporting ceramic matrix composite body having a configurationwhich replicates the configuration of said at least one preform.
 16. Aself-supporting ceramic matrix composite body consisting essentially offrom about 2% to about 95% by volume of a three-dimensionallyinterconnected ceramic matrix and from about 5% to about 98% by volume,based on the total volume of the ceramic matrix composite body, of atleast one preform contained within said ceramic matrix, said preformbeing a shaped body of filler material having a configuration which iscapable of supporting its own weight and maintaining dimensionalfidelity and interconnected porosity without any means of supportlocated external to any surface of said preform, said ceramic matrixbeing disposed within at least a portion of said interconnected porosityso as to embed the filler material and said ceramic matrix consistingessentially of, based on the total weight of said matrix, about 60% toabout 99% by weight of an interconnected oxidation reaction productconsisting essentially of a material selected from the group consistingof alumina, aluminum nitride and silicon nitride, and the remainder ofsaid ceramic matrix consisting essentially of a metallic constituent andvoids, said self-supporting ceramic matrix composite body having aconfiguration which replicates the configuration of said at least onepreform.
 17. The ceramic matrix composite body of claim 1, wherein saidat least one preform comprises an assemblage of preforms.
 18. Theceramic matrix composite body of claim 15, wherein said at least onepreform comprises an assemblage of preforms.
 19. The ceramic matrixcomposite body of claim 16, wherein said at least one preform comprisesan assemblage of preforms.